In year's past, a good rule of thumb for trading up into a round was that it would cost you picks in the two subsequent rounds. So to trade into the second, you'd probably trade your third and fourth rounders. But again, it depends on how early the pick is in that round.
And you have different teams willing to pay different prices for things. And when players are involved in the trades, it can completely throw off value.
Trading future picks generally devalues them by a full round on the TVC,